1. Field of the Invention
The present invention relates to a field-effect-type liquid crystal display device to be time-division driven (multiplex-driven).
2. Description of the Related Art
A liquid crystal display device is widely used as a display device of office equipment. A liquid crystal display device with a large number of pixels and high display quality is requested according to enlargement of the display screen size. A liquid crystal display element consisting of a single matrix structure allowing the element structure and drive unit to be simplified is widely used for the office equipment.
The liquid crystal display element consisting of the single matrix structure has one substrate in which a signal electrode is formed like a stripe, another substrate in which a scanning electrode is formed like a stripe so that the signal and scanning electrodes cross each other, and a liquid crystal cell consisting of a twisted liquid crystal member sandwiched between both substrates. A pair of polarizing plates is installed outside the liquid crystal cell. The pixel formed on the portion where the scanning and signal electrodes are crossed is turned on/off by supplying a scanning signal to the scanning electrode and a data signal to the signal electrode synchronously with the scanning signal.
For the time-sharing driving method, a stroke is produced between turned-on and turned-off pixels. Therefore, if the number of time divisions for time-division driving is increased, the difference of the voltage applied to the turned-on and turned-off pixels or the operating margin decreases, the contrast is impaired, and the view angle narrows. The conventional TN-type liquid crystal display element only takes the number time divisions of up to the duty ratio of approx. 1/60 because it is inferior in the sharpness of brightness change for the applied voltage. Therefore, it is difficult to obtain a display device with a high quality and large size.
To improve the sharpness, STN-type and SBE-type liquid crystal display elements with a large twist angle are practically used. These liquid crystal display elements can take the number of divisions of up to the duty ratio of approx. 1/200 when the sharpness is improved by increasing the twist angle to 180.degree. to 270.degree..
Therefore a liquid crystal display element with 640.times.400 dots uses the method in which a display screen is divided into upper and lower groups and each group is driven by the duty ratio of 1/200. The liquid crystal display element with upper and lower groups divides the signal electrode into upper and lower portions at its center. Therefore, the area of the pixel adjacent to the boundary between two divided portions changes depending on the substrate alignment accuracy and the line to divide the screen appears. In addition, because the signal electrode is divided into upper and lower portions, the number of driving circuits for supplying the data signal to the signal electrode requires two times as many as the number of signal electrodes. Also, the driving circuit of the scanning electrode for supplying the scanning signal to the scanning electrode requires complicated signal processing to execute scanning by dividing the scanning electrode into two groups. Therefore, there are disadvantages that the size of the display device increases because the driving circuit becomes complicated, the number of elements of the electronic circuit increases, and the device cost increases. Moreover, the above liquid crystal display element has disadvantages in view of the operation stability that the response time is too long because of a large twist angle and display irregularity occurs due to a long time operation.